A Command, Telemetry, and Positioning System for Spacecraft Using GPS Tkansdigitizers

William S. Devereux and Dennis J. Duven

Abstract:	A new spacecraft command, telemetry, and positioning concept has been developed that offers significant advan- tages over present systems. The proposed system uses a significantly smaller and lighter-weight flight element that is equivalent to present standard coherent transponders, and employs simplified ground stations with reduced overall mission operations costs. It uses the Global Positioning System (GPS) to perform the positioning function and thus is called the GPS-Linked Transponder (GLT) Command, Telemetry, and Positioning System. The GLT is the flight element and comprises a NASA STDN/DSN-compatible or USAF/AFSCN-compatible command receiver/detector, a 20-Mbps-capable PCM/PSK telemetry transmitter with a selectable-rate FEC encoder and optional embedded encryptor, and a dual-mode spacecraft positioning sub- system including a GPS transdigitizer and, optionally, a full GPS receiver/navigator. The use of a GPS-based system eliminates the need for coherent turnaround of uplink carrier and ranging signals and associated ground support. In the transdigitizer-only mode, position andvelocity of the space- craft are computed within seconds of ground data reception using techniques developed at APL for missile test pro- grams. Accurate spacecraft orbital ephemeris elements are computed after one pass over a single ground station. The primary advantages claimed are a significant reduction in mass, size, and complexity for the spacecraft transponding system, leading to higher inherent reliability, reduced costs, and added flexibility for spacecraft designers. Mass and size are reduced to nearly one-tenth those of existing transponder systems. Other advantages include significantly higher data rate uplink and downlink communications than presently supported. Ground-station operating costs should also be significantly reduced. A GPS transdigitizer, when used as an embedded subsystem of a telemetry transmitter/command receiver system, is shown to be the optimal approach for positioning a large class of spacecraft. Several ground- station configurations are defined covering a range of achiev- able spacecraft positioning accuracies. Accuracy analysis techniques are described, and predicted uncertainties for each configuration are provided.
Published in:	Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993) September 22 - 24, 1993 Salt Palace Convention Center Salt Lake City, UT
Pages:	279 - 291
Cite this article:	Devereux, William S., Duven, Dennis J., "A Command, Telemetry, and Positioning System for Spacecraft Using GPS Tkansdigitizers," Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993), Salt Lake City, UT, September 1993, pp. 279-291.
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